I have been doing a lot of thinking towards my next cannon, which I plan to be an over/under (i.e., non-inline), 2 inch diameter barrel and 2 inch diameter combustion tank, both of equal length (approx. 36”). The weapon would be held and fired like a SMAW. Recently, I have been wrestling with what to build it out of. Below is a discussion of what I have considered. I invite anyone to post their comments, suggestions, or responses to some of the questions that will arise in the discussion.
PVC
Tried and true, I could go this route. My main concern is that I will not be able to complete my design with this material. What is the most reliable way to join two parallel lengths of PVC together? Would you recommend normal PVC glue and some kind of constructed separator (such as is used on extended magazines on pump/semi-auto shotguns)? This question applies to the other materials as well, although welding is an option on the metals. My secondary concerns are strength in wear and tear and the overall mediocrity of a PVC spud gun: everyone has made one.
Aluminum
With the connections I have to make, I would either have to weld it or get fittings. Does anyone know where to get 2” aluminum pipe fittings? What wall thicknesses should I use for a normal combustion gun (I am assuming I would be using 6061)? The downsides to this material are that I would have to get some welding equipment. I have an argon/CO2 tank and wire for steel welding and a MIG welder. I could go out and get a pure argon tank and some aluminum wire, but would it be worth it?
Plain steel (not normal black, no seams)
I could weld this stuff, but how thick (and therefore heavy) would plain (i.e., mild, non seamed) steel pipe have to be to hold the kinds of pressures (I would be using propane as a fuel, I assume there is some maximum pressure for this type of combustion. Was it 150 psi?). I want to avoid an overly heavy gun, so building in schedule 40 steel would be out of the question. What could I safely get away with?
Also, what would I ask for? I am not very familiar with the terminology used in steel piping for this kind of stuff. I would be planning on coating the whole thing in black hockey tape to give it a good look and feel and to ward off corrosion. If I wanted better corrosion resistance, what type of paint should I paint it first with (I assume it would be something other than a generic oil based paint)?
Stainless steel
Not sure what it takes to weld this stuff, but it is expensive. It would cost $200+ to build one cannon out of this, not taking into account anything but the piping itself. Would I be using 304 or a different alloy?
One final note on this gun: I was hoping to build single shot canisters filled a high pressure mix of propane/air. I am not exactly sure how to calculate the right mix (I did read http://www.spudfiles.com/forums/hybrid- ... 13602.html and I did watch , but I have been unable to combine these into a useful way of deciding where the needles on the pressure gauges should be after the propane injection and then of the HPA injection). Perhaps someone could write HYBRID FUELING 201: Applications
. These canisters would contain enough fuel to fill my chamber and then some (they would inject via a male/female Schrader valve coupling on the canister/combustion tank, respectively). I was thinking somewhere between a 1.5x and a 2x chamber volume equivalent shoved into a small canister. My question is this: if I were to use black steel canisters, how big (and therefore what true mix (10x, 20x…?) they would contain) should I make them to avoid a rupture if one of them accidentally ignited during fueling?The fueling process would be something like this: attach the propane tank to the fuel meter (described in “how to build a propane meter” video), inject fuel until the needle gets to the right point. Inject air from an HPA tank until the needle gets to the right point (again, I don’t know what these points are, or how to find them). Then open the injection valve and fill my canisters. Repeat this process for all canisters. Press a given canister into the front of the combustion chamber (where the female Schrader valve is) and allow it to fill the chamber. Fire away. Has anyone else done this?
After I work out the final design I will post it (as it does not depend heavily on what material I will be making it out of).
Cheers
The barrel is exhaust pipe, not the chamber, and it was 300psi.
